Antibiotic resistance in bacterial and other pathogens is becoming increasingly important. It is therefore important to find new therapeutic approaches to attack pathogenic microorganisms.
Pathogenic microorganisms have to evade the host's defense mechanisms and be able to grow in a poor nutritional environment to establish an infection. To do so a number of “virulence” genes of the microorganism are required.
Virulence genes have been detected using classical genetics and a variety of approaches have been used to exploit transposon mutagenesis for the identification of bacterial virulence genes. For example, mutants have been screened for defined physiological defects, such as the loss of iron regulated proteins (Holland et al, 1992), or in assays to study the penetration of epithelial cells (Finlay et al, 1988) and survival within macrophages (Fields et al, 1989; Miller et al, 1989a; Groisman et al, 1989). Transposon mutants have also been tested for altered virulence in live animal models of infection (Miller et al., 1989b). This approach has the advantage that genes can be identified which are important during different stages of infection, but is severely limited by the need to test a wide range of mutants individually for alterations to virulence. Miller et al (1989b) used groups of 8 to 10 mice and infected orally 95 separate groups with a different mutant thereby using between 760 and 950 mice. Because of the extremely large numbers of animals required, comprehensive screening of a bacterial genome for virulence genes has not been feasible.
Recently a genetic system (in vivo expression technology [IVET]) was described which positively selects for Salmonella genes that are specifically induced during infection (Mahan et al, 1993). The technique will identify genes that are expressed at a particular stage in the infection process. However, it will not identify virulence genes that are regulated posttranscriptionally, and more importantly, will not provide information on whether the gene(s) which have been identified are actually required for, or contribute to, the infection process.
Lee & Falkow (1994) Methods Enzymol. 236, 531-545 describe a method of identifying factors influencing the invasion of Salmonella into mammalian cells in vitro by isolating hyperinvasive mutants.
Walsh and Cepko (1992) Science 255, 434-440 describe a method of tracking the spatial location of cerebral cortical progenitor cells during the development of the cerebral cortex in the rat. The Walsh and Cepko method uses a tag that contains a unique nucleic acid sequence and the lacZ gene but there is no indication that useful mutants or genes could be detected by their method.
WO 94/26933 and Smith et al (1995) Proc. Natl. Acad. Sci. USA 92, 6479-6483 describe methods aimed at the identification of the functional regions of a known gene, or at least of a DNA molecule for which some sequence information is available.
Groisman et al (1993) Proc. Nat. Acad. Sci. USA 90, 1033-1037 describes the molecular, functional and evolutionary analysis of sequences specific to Salmonella. 
Some virulence genes are already known for pathogenic microorganisms such as Escherichia coli, Salmonella typhimurium, Salmonella typhi, Vibrio cholerae, Clostridium botulinum, Yersinia pestis, Shigella flexneri and Listeria monocytogenes but in all cases only a relatively small number of the total have been identified.
The disease which Salmonella typhimurium causes in mice provides a good experimental model of typhoid fever (Carter & Collins, 1974). Approximately forty two genes affecting Salmonella virulence have been identified to date (Groisman & Ochman, 1994). These represent approximately one third of the total number of predicted virulence genes (Groisman and Saier, 1990).
The object of the present invention is to identify genes involved in the adaptation of a microorganism to its environment, particularly to identify further virulence genes in pathogenic microorganisms, with increased efficiency. A further object is to reduce the number of experimental animals used in identifying virulence genes. Still further objects of the invention provide vaccines, and methods for screening for drugs which reduce virulence.